1. Field of the Invention
The present invention relates to a dichroic crystal which can exhibit a high dichroic property and which can be applied to a variety of electronics devices and optical devices, e.g., polarizing plates, liquid crystal display plates and suspended particles display (i.e., SPD) devices.
2. Description of Related Art
There has been a polarized film, one of the optical devices, which is made by absorbing a dichroic material, such as a dichroic pigment having a slender molecular structure or iodine, in a polymer matrix like polyvinyl alcohol (i.e., PVA) and followed by elongating the polymer matrix. The conventional dichroic materials exhibit a high dichroic ratio, but it is slightly poor in terms of heat resistance. As a result, electronics and optics engineers have longed for a dichroic pigment which can not only produce a high-grade dichroic property but also exhibit a high heat resistant.
Phthalocyanine is stable against light and heat, and is superior in fastness of color. Since it has conjugated pielectrons in its molecule, it has a large variety of interesting electronic and optical characteristics so that it has drawn attention as a material for electrophotographically sensitive substances, solar cells, optically recording devices.
However, it is less likely that phthalocyanine is used as a dichroic material. This results from the fact that it hardly forms a crystal in which all of the phthalocyanine molecules are aligned in parallel. For instance, in the case of copper phthalocyanine crystals, the alpha-crystal has the molecular plane inclining by 26.5 degrees, and the beta-crystal has the molecular plane inclining by 45.8 degrees. Hence, the molecules of the copper phthalocyanine have transition moments which are confined in the molecular planes. As a result, the copper phthalocyanine has a great dichroic property inherently, but its crystals exhibit a small dichroic property.
In order to use such phthalocyanine as a dichroic pigment, it is necessary to control the molecular alignment of the phthalocyanine molecules in crystals so as to effect the polarization. There have been several methods which can control the molecular alignment of phthalocyanine. For example, Japanese Unexamined Patent Publication (KOKAI) No. 3-262,639 sets forth one of the methods, and it discloses a process for preparing an LB film having a dichroic property by using phthalocyanine substituted with an alkyl group.
According to the Journal of American Chemical Society, 1980, volume 102, pages 6,702-6,713, Marks et al. disclose another method for controlling the molecular alignment of phthalocyanine. Namely, they synthesized nickel phthalocyanine iodide, and they examined the resulting single crystals for their structures by means of the X-ray diffraction microscopy and found out that the iodide crystals have a structure where iodine intrudes into the spaces between the columns of the laminated nickel phthalocyanine. As a result, it is believed that the crystalline structure of phthalocyanine can be modified by forming iodide complex.
However, phthalocyanine is generally obtained in a form of poly-crystalline particles. For instance, it is prepared in a form of aggregate in which micro-fine crystals having an average particle diameter of hundreds of angstroms are aggregated and whose molecular planes are aligned in different directions as a whole. As a result, when phthalocyanine being insoluble to a solvent is employed, the resulting phthalocyanine iodide is prepared in a form of poly-crystalline particles. In the case where the phthalocyanine particles are used as the dichroic pigment, there arises the dichroic phenomenon only when the crystalline articles are single crystals having an average particle diameter of from a few micrometers to a few millimeters. In other words, only when such single crystals are employed therefor, the transmissivity of lights having a predetermined wavelength band can be changed considerably depending on the direction of the polarization planes of the lights with the single crystals. However, when the crystalline particles are poly-crystalline ones, the individual crystalline particles do not exhibit the dichroic property. Thus, it has been difficult to readily make the conventional phthalocyanines into crystalline particles each of which is capable of exhibiting the dichroic property.